The present invention relates to elastic surface wave transducers making it possible to convert the difference between two pressures into the frequency of an electric signal. Such a transducer has two tubes at the inlet of which appear the fluid pressures to be measured. A thin plate of piezoelectric material within the transducer elastically deforms under the pressure of the fluids. One of the faces of this plate carries four sets of electrodes in the form of interdigitated transducers in such a way as to form two electromechanical delay lines. The delay lines cooperate with two amplifiers so as to form two electrical oscillators, whose frequencies vary as a function of the deformations undergone by the plate. The transducers which form in pairwise manner the delay lines exchange elastic surface waves in two surface area of the plate where, for the same mechanical stress, oppositely directed actions occur on the travelling times. By connecting the two oscillators to a subtractive mixer, a measuring signal is obtained, whose frequency represents the variation between the pressures to be measured. It is known to uniformly apply the two pressures to be measured on either side of the piezoelectric plate, which supplies a pressure equal to the difference of the acting pressures. In this case, the exchange areas of the elastic surface waves are arranged asymmetrically in order that the frequencies of the oscillators vary in opposite directions and their effects are added after subtractive mixing.
This solution has a number of disadvantages. One of the fluids is in contact with the face of the piezoelectric plate which propagates the elastic surface waves. There can be significant disturbance to the propagation of elastic surface waves if the gas contains moisture and dust. The asymmetric positioning of the delay lines means that it is not easy to effectively compensate thermal drift.